The straps on higher-quality respirators are not just elastic, they’re some kind of finely braided textile that doesn’t snag hair. The sole difference between the 3M 8210 and the 3M 8210+ is whether it has these more expensive straps.
As for the serial numbers: hmm, maybe that could help, but if each number is unique and 1/10th of them actually get checked, then the counterfeiter can just copy a bunch of unique ones and go undetected 99% of the time. (1/10th of the good masks and 1/10th of the counterfeits would be checked, and a detected duplicate would have to be in the intersection of those two sets). In fact if good masks come from known and trusted suppliers, they're even less likely to be checked, so more of the counterfeits can get through.
What we really need is an affordable way to test masks at home. It doesn't have to be super precise. A green-yellow-red led for good-marginal-bad would be fine.